Heat shock proteins have been reported to confer resistance to certain antineoplastic drugs. We investigated the impact of hsp70 overexpression on the efficacy of two new anti-cancer drugs, topotecan and gemcitabine. We used the fibrosarcoma WEHI-S cells stably transfected to overexpress the hsp70 cDNA from the constitutive SV40 promoter and appropriate control cells. After topotecan and gemcitabine treatment hsp70-overexpressing cells showed a marked elevation in cell survival, suggesting that hsp70 overexpression was sufficient to confer resistance to the drugs. In addition, hsp70-overexpressing cells were capable of starting cell proliferation after treatment with drug dosages that were lethal to control cells. Our results demonstrate that hsp70 overexpression represents a possible cause of drug resistance. In order to transfer these data to tumour cells constitutively expressing stress hsp70 due to the constitutive activity of the original hsp70 promoter we sought to supress the heat shock response pathway by the natural flavonoid quercetin, known to inactivate the heat shock transcription factor (HSF). Using a suitable cell line, we demonstrated the sensitising activity of quercetin. We found that antineoplastic drug concentrations exerting cytotoxic activity were markedly lower when cells were pretreated with quercetin. Concomitantly, hsp70 expression was strongly down-regulated under quercetin treatment. Our data indicate that quercetin may be useful as a sensitiser in chemotherapeutically treated patients suffering from hsp70-overexpressing tumours.